Preheat the oven to 350°. Prepare two 6 inch pans or one 9 inch pan by lightly coating with butter and then 'flouring' the pan with a little cocoa powder. Set aside.
Using the whisk attachment on a stand mixer, beat together the melted butter and sugar. Beat for 3 minutes, until light and fluffy.
Add in the eggs, egg yolk and vanilla. Mix until well combined.
In a small bowl, whisk together the milk and sour cream until smooth. Add to the batter (butter/sugar/egg mixture). Mix until well combined.
Add in the flour, cocoa ,baking powder and salt. Slowly mix until smooth.
Pour the batter into prepared pan(s).
Bake at 350° for 25-30 minutes or until a toothpick comes out clean.
Cool completely before removing from pan or cutting.